1. Field of the Invention.
This invention relates to polymer composites and more particularly to sound attenuating polyurethane composites.
2. Description of the Prior Art.
Many compositions and processes have been proposed for attenuating sound including the use of various polymer composites. However, there have been numerous difficulties associated with the use of the different composites. Cross-linked rubber composites, for instance, require large, expensive processing machinery. In manufacturing certain sound attenuating composites, such as those made from thermoplastics, there are high internal mold pressures and the polymers must be processed in high pressure processing equipment including very strong molds and special clamping mechanisms. Alternatively, the polymers are processed separately in sheet form and adhered to the end product. Thermoplastic type composites are described in, for example, U.S. Pat. No. 4,241,806.
Some sound attenuating composites use physical or chemical blowing agents to form voids which dissipate acoustic energy as sound waves travel through the composites. However, the use of chemical and physical blowing agents creates gas generated pressures, which can cause large internal forces within confining molds. Consequently, very strong molds and expensive clamping mechanisms are necessary to contain such internal forces.
Two-part polyurethane composites are recognized in the art for energy-absorbing properties. Many of these polyurethanes use a 2000 molecular weight polytetramethylene ether glycol which crystallizes at lower temperatures and produces an undesirable abrupt increase in modulus. In certain instances the composites of the prior art are flammable and have burning characteristics which promote the spread of fire.
Thus, there is a need for a polyurethane binder which does not crystallize at lower temperatures such as, e.g., a temperature range of about -10.degree. C. to 70.degree. C.
There is also a need for a composite having excellent damping properties at lower temperatures than most polymers.
There is a need for a composite which may be processed with relatively inexpensive processing equipment such as standard low pressure processing equipment. Since it is desired that no pressure be developed inside the mold while forming the expanded polymer, it would be desirable to provide an expanded composite without using gas generating substances in the polymer.
There is also a need for a composite which has superior resistance to flame propagation and superior non-drip burning characteristics, which inhibit the spread of a fire.
Finally, there is a need for a polyurethane polymer formulated such that the acoustic energy dissipation mechanism of the polymer is optimized, at the required temperature of use and frequency of generated sound.